Subfloor over concrete questions


Old 04-22-05, 09:31 AM
Visiting Guest
Posts: n/a
Subfloor over concrete questions

I have an attached garage that I am converting into living space. As a background: The garage slab is at grade, then above grade since the driveway outside runs next to it and slopes down toward the street. It has never gotten wet from water running in, leaking etc, except for condensation.

My evil plan is to lay 2x4' on side, use 1 1/2" foamular insulation (pink closed cell board stuff), 6 mil poly, 3/4" sturdi board (i like plywood to but this stuff is bullet proof with water) then hardwood on top. 2x4s' anchored with 3 1/4" x 1/4 tapcon + construction adhesive.

1st question- What spacing for the screws should I use? 5 per 8' board?
2nd question- I want to put the vapor barrier above the insulation and below the 3/4" OSB. My reasoning is that if I put it on the cold concrete surface it will cause the vapor to condense there and get trapped. Just as in your wall design the vapor barrier goes to the warm side of the house, not the exterior sheathing. If the concrete was sucking up water, I can see the need to place a vapor barrier under the 2x4's.
3rd question- I really want to avoid PT. due to the out gassing. I was thinking of regular fir studs treated with a wood preservative or Bin (exterior grade shellac) but I think I'm going to cave on this one.

3A- Decided to got with PT- Now which to use? There are 4 grades of chemical treatment. Above ground- ACQ .25? Ground contact- ACQ .4?
Or even better?



Last edited by spanky; 04-25-05 at 04:28 AM. Reason: Adding question
Sponsored Links
Old 05-02-05, 12:37 PM
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 17,763
1. I would space my fasteners about 6-8" apart.

2. I tend to think you will get more moisture from the concrete than the room, so I'd put the plastic down first and put everything on top of it.

3. Use pressure treated any time it contacts masonry. I would use one approved for ground contact. I don't see a way around this one. You will have a minimal amount of treated lumber and it will be entombed below the floor and any gassing will be minimal. If anyone in your house smokes the wood offgassing should be the least of your worries.
Old 05-02-05, 02:44 PM
Join Date: Feb 2005
Location: USA
Posts: 339
I suggest you look at Dricore.

I used it in a similar situation and then put laminate flooring over it. It is easy to install and my project worked out really well.
Old 05-13-05, 08:50 PM
Join Date: Feb 2005
Posts: 67
american plywood ass'n reccomend floor sheating attachment at 6" at sheet edges and 12" at interior supports

dont get hung up on the water/slab/vapor barrier issue. this is not an exterior wall condition. temperature and humidity differences will find a happy medium and will not build water. dont sweat it (no pun intended)

not sure what you mean by outgassing. check your local lumber yards for borate treated wood. it is an ill looking blue and is designed for interior use as the borate treatment is water soluble (i cant even begin to explain that)

if you have to use ACQ, the minimum treatment is more than adequate. remember that ACQ is 3 times more corrosive on steel so you need to use hot dip galvanized or stainless fasteners.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes